The present invention relates to a sheet having a multilayer structure, called a sandwich sheet, composed of two rolled metal skins joined together by a core. A process for making a multilayer structure also makes up part of the invention.
Industrial sectors, especially the motor-vehicle industry, require lightweight materials, such as, for example, steel composite materials of the multilayer type, which have the properties of steel but a lower density. These multilayer materials may be used especially in the manufacture of motor-vehicle body components or of exhaust lines.
Many patents describe multilayer structures consisting of two metal skins combined with a core made of organic resin.
For example, Patent FR 90/06805 relates to a process for producing a sheet having a multilayer structure, called a sandwich sheet. Here, the sheet is obtained by a process being composed of two metal skins and a core made of polymer material, the purpose of which is to provide an acoustic or vibration damping function or to improve the stiffness-to-weight ratio. Forming this type of multilayer structure poses joining and welding problems. This is because the poor mechanical properties of the material of which the core is composed causes the material to creep during drawing. From the welding standpoint, the material of which the core is composed is an electrical insulator, thereby causing a problem in terms of conductivity in the thickness direction during welding, even when, in practice, the conductivity is improved by adding electrically conductive fillers, such as nickel balls, for example, to the material.
Patent FR 89/17142 describes a structure composed of two covering outer skins connected together by a low-density core, this core consisting of a material which is less rigid than the skins, such as, for example, a textile core hot-formed after being impregnated with a synthetic resin. The technique described in this document makes it possible to produce a composite product having a sandwich structure, of any surface area and shape, which includes regions of particular mechanical strength or of a specific property such as, for example, an acoustic, vibratory or thermal property. In the process presented in this document, the core is formed prior to the two skins being put into place, which skins are also formed before assembly.
One object of the present invention is to avoid the drawbacks of the structures described above, by providing a sheet having a multilayer structure, called a sandwich sheet, composed of two rolled metal skins which are joined together by an electrically conductive core, it being possible for the forming of the sheet to be carried out as in the case of a simple, single sheet.
One subject of the invention is a sheet having a multilayer structure, called a sandwich sheet, composed of two rolled metal skins which are joined together by a core of stainless steel wool.
Other characteristics of the invention, which may be present individually or in any combination, are:
the core consists of, consists essentially of, or comprises stainless steel wool;
the stainless steel wool occupies from 30% to 60% of the volume separating the two skins;
the skins are made of steel chosen from stainless steel, carbon steel, aluminized stainless steel and aluminized carbon steel;
the steel wool is a ferritic stainless steel wool;
the core and the two skins are joined together by welding;
the welding is carried out by capacitor-discharge arc welding;
the two skins are joined to the core by means of an adhesive;
the adhesive is chosen from a phenolic resin, an epoxy, and a polyethylene or polypropylene containing maleic anhydride.
The invention also relates to a process for producing a sheet having a multilayer structure, called a sandwich sheet, composed of two rolled metal skins joined together by a core of a stainless steel wool, in which process:
the two skins and the core are assembled by calendering, so as to define the thickness of the multilayer structure;
an arc is generated perpendicular to the surface of the skins in order to weld, in the arc zone, on the one hand, the fibers of steel wool which are in contact with each other, in the core, thereby forming a rigid network, and, on the other hand, the fibers of steel wool which are in contact with the internal surfaces of the two skins in order to ensure connection between the core and said skins.
The invention also relates to the use of a sheet in an application involving the production of a tube for a motor-vehicle exhaust line in which the sheet having a multilayer structure is rolled and welded, for example by induction welding, in order to form a tube having a double wall and a core of stainless steel wool.